Exciplex Emission of Carbazole–Terephthalate Cyclophane and Its Application as a Fluorescent Probe for Local Polarity of Solvents

Photophysical properties of carbazole–terephthalate cyclophane (Cz–TP) were studied. The absorption spectra of Cz–TP in various solvents were slightly broadened due to the distortion of the carbazole (Cz) moiety and became similar to that of a reference compound of carbazole–benzene cyclophane (Cz–Bz). The absorption spectrum and the peak wavelengths of Cz–TP were insensitive to the solvent polarity. These findings show that there is transannular π–π electronic interaction between the Cz and TP moieties but that the charge transfer interaction from the Cz to TP moieties is small in the ground state. On the other hand, the emission spectrum was broad, structureless, and red-shifted compared with that of the reference compound. The peak wavelength of the emission was shifted to a longer wavelength with increases in the dielectric constant of solvents. The peak shift was quantitatively explained by the Lippert–Mataga formula. Therefore, we conclude that the broad emission is due to the intramolecular exciplex in Cz–TP. The dielectric constant of 2-methyl tetrahydrofuran was evaluated from the exciplex emission of Cz–TP ranging from 77 to 292 K, indicating that the Cz–TP molecule is an excellent polarity probe not only in a liquid phase but also in a solid phase over a wide temperature range.